Within socially monogamous species, a male's reproductive success depends on his success at obtaining a social partner with which he mates, as well as gaining extra-pair matings. We investigated the impacts of body mass and parasitism on 2 measures of a male's success at obtaining a social partner (number of female social associations and relative strength of a social bond to a single female) and 2 measures of reproduction (number of females with which a male sires offspring and the number of offspring sired) in natural populations of prairie voles (Microtus ochrogaster) located in Kansas and Indiana. Kansas males with greater endoparasite loads were less likely to have one social partner. Instead, they associated with multiple females although they did not sire offspring with more females than males with lower levels of endoparasitism. We did not find any relationship between endoparasite infestation and the number of females with which males associated in Indiana. There was no association between the level of endoparasites found among males in either Indiana or Kansas and the strength of a male's social bond to a female. Endoparasites, at least at the infection levels detected in this study, were not related to indices of male reproductive success in Microtus populations in Kansas or Indiana. No relationship was found between body mass and indicators of social monogamy in either population. However, body mass appears to be significantly related to male reproductive success. Heavier males produced offspring with more females, particularly in the Kansas population, and sired more pups in both populations.